Solid-state imaging devices, such as CMOS sensors, have been used in a wide variety of applications, including digital still cameras, video cameras, and surveillance cameras. Recently, a back-illuminated solid-state imaging device has been proposed to suppress a decrease in the signal-to-noise ratio due to a decrease in the pixel size. In the device, light entering the pixels can reach a light-receiving region formed in the Si layer without being impeded by the interconnection layer. Therefore, high quantum efficiency can be achieved in a minute pixel.
In the back-illuminated solid-state imaging device, incident light entering the light-receiving region in the Si layer generates photoelectrons. Part of the photoelectrons leak into adjacent pixels by thermal diffusion, causing crosstalk. To reduce crosstalk, a p-well (pixel separation region) is provided between adjacent pixels to produce a diffusion potential difference, thereby suppressing the diffusion of photoelectrons.
However, such a structure has the following problem. The pixel separation p-well is formed by selectively implanting B (boron) ions at high energy from the front side of the Si substrate by use of a mask. At this time, the width of the p-well must be made extremely narrow. Therefore, the ion implantation mask must be so made that its aperture width is narrow and it is thickened to such a degree that it can withstand high energy. However, it is difficult to form a thick mask with a narrow aperture. The mask material sometimes stays behind on a part of the narrow aperture. In this case, the p-well is not formed as far as the light incident surface of the Si substrate, permitting crosstalk to occur due to the diffusion of photoelectrons. As a result, color mixture increases on the replay screen and therefore only reproduced images with deteriorated color reproducibility are obtained.
In addition, the pixel separation p-well formed by ion implantation from the front side of the substrate extends due to a scattering of ions at the back side of the substrate. The extension of the width of pixel separation on the light incidence side becomes a factor that increases crosstalk.